You can see a non-vented one if you click on the gallery view and it's a few posts down.
I've only tried the vented strap on the RM10. Comfort wise, it's about the same as the other straps for me.
All RM straps feel really good on the wrist. The only problem with comfort nowadays is the deployant. There are some sharp edges that get very annoying and I end up using a watchmakers file to file down some of those parts on the deployant. I think the rubber strap only changes the look of the watch not the comfort level.